Chapter 48
Exercise in the Heat

with Timothy Noakes

*This Chapter is online in its entirety. Click on the links below to browse through the whole chapter.*

Mechanisms of heat gain and loss
Clinical Perspective
Heatstroke - a temperature above 41ºC
Exercise-associated collapse
Cramps

Because sporting activity may occur in hot conditions, the sports medicine practitioner must be well versed in both prevention, and management, of heat-associated illness. Humans can only survive core temperatures greater than 41 degrees C (106 degrees F) for short periods and protein denatures at a body temperature of 45 degrees C (113 degrees F). 

Certain individuals are at greater risk than others for heat injury. These include the obese, the unfit, the very young and the very old and those unacclimatized to heat. The presence of a fever, commonly due to a viral infection (e.g. gastroenteritis or upper respiratory tract infection), may also increase the risk of heat illness.

In this chapter we briefly review the physiology of heat gain and heat loss before discussing the diagnosis and management of three common presentations in the heat - heatstroke, exercise associated collapse and cramps. As these names suggest, only the former is truly related to heat! Finally, we discuss an important differential diagnosis of heatstroke - hyponatremia. This condition results from fluid overload.

Mechanisms of heat gain and loss

Heat is produced by both endogenous sources (muscle activity and metabolism) and exogenous sources (transfer to the body when environmental temperature exceeds body temperature). The rate of heat production and the risk of heat stroke are greatest in those who run the fastest and have the highest work rate, i.e., in short-distance rather than marathon events. Heat loss occurs by conduction, convection, radiation or evaporation. At rest, when environmental temperature is below body temperature, thermal balance is maintained by convection of heat to the skin surface and radiation of heat to the environment. 

As an individual starts to exercise and produce more heat, sweating provides compensatory heat loss through evaporation. When the environmental temperature equals or exceeds body temperature, sweating is the predominant mechanism of heat loss [1]; athletes exercising in these conditions rely almost exclusively on evaporative heat loss to regulate body temperature. 

The effectiveness of sweating to cool the body is affected by humidity. In a dry environment, sweat is evaporated. A humid environment, where there is a high level of water vapor in the air, limits evaporation of sweat and its cooling effect. Therefore, the combination of high temperature and high humidity is particularly dangerous. Athletes should avoid exercising in these conditions if possible. In extreme environmental conditions, core temperature can increase substantially even in relatively short-distance races (6-15 km). 

The effects of humidity as well as the effects of solar and ground radiation, air temperature and wind speed are included in the wet bulb globe temperature (WBGT) index. This index is used to determine the amount of activity that should be undertaken in hot conditions. It is recommended that endurance events should not be held when the WBGT index exceeds 28 degrees C (82 degrees F) [2]. 

Clinical perspective

Clinical assessment of the athlete who presents after exercising in the heat is the cornerstone of good management. In the past, there has been a tendency to initiate treatment before making a rational diagnosis. This position was taken partly because of the problem of high rates of admission to the medical tent at major events and partly because it was assumed that all athletes who collapsed were dehydrated and needed immediate intravenous hydration. The former problem can be overcome by a system of triage at events (Chapter 54) and the latter assumption is not evidence-based [3]. Thus, the emergency treatment of life-threatening conditions, including heatstroke and hyponatremia, can safely be delayed for 1 or 2 minutes while the rectal temperature is measured and a reasonable working diagnosis is established. The obvious exception is cardiac arrest, which occurs uncommonly, and the diagnosis of which is unambiguous. 

Criteria for determining the severity of collapse are shown in Table 48.1. The initial assessment is based on the athlete's level of consciousness, and knowledge of where in the race the athlete collapsed. Patients who are seriously ill show alterations in their level of consciousness and almost always collapse before completion of the race. 

Table 48.1. Guidelines for determining the severity of the collapsed athlete's condition. 

Measuring rectal temperature, blood pressure, and heart rate provides additional diagnostic information. In longer races (>25 km) when hypoglycemia is more likely, a glycometer should also be provided. In mass events of much longer duration (>4hr), including ultramarathons, equipment for measuring serum sodium concentration must be available so that potentially lethal exercise-related hyponatremia can be diagnosed expeditiously. Intravenous therapy should only be considered after a serum sodium concentration >135mmol/l has been demonstrated. 

Whether or not the athlete is conscious or unconscious makes a great difference to the differential diagnosis. If the athlete is unconscious, the initial differential diagnosis is between a medical condition not necessarily related to exercise, for example, cardiac arrest, grand mal epilepsy, subarachnoid hemorrhage, or diabetic coma, and an exercise-related disorder, most especially heatstroke, hyponatremia, or severe hypoglycemia. The latter is an uncommon cause of exercise-related coma in non-diabetic subjects. If the athlete is unconscious, the crucial initial measurement is the rectal temperature, followed by heart rate and blood pressure. If the rectal temperature is >41 degrees C the diagnosis is heatstroke, and the patient must be cooled immediately (see below). 

If the rectal temperature is <40 degrees C in an unconscious patient, the blood pressure and pulse rate are not grossly abnormal, and there is no other obvious medical condition, the probability is that the athlete has hyponatremia of exercise, or rarely, another electrolyte abnormality such as hypochloremia also causing cerebral edema. Note that 'dehydration' in the range measured in athletes does not cause unconsciousness.